Combustive products from municipal refuse can be very corrosive. During the operation of refuse-to-energy facilities, it is important to protect boiler tubes from corrosion. Corroded boiler tubes can leak which reduces the facility efficiency and frequently leads to the premature shut-down of the facility. Prior facilities provided arrangements to protect boiler tubes, however, these arrangements have been costly and have not been proven to be effective in protecting boiler tubes over prolonged periods of time.
The need to provide environmentally correct and cost effective solutions for the refuse generated in the Unites States became apparent in the late 1960's. At that time, refuse disposal was mainly by land filling and to a lesser extent incineration. That situation changed as landfill space became recognized as a finite resource and that refuse could be used as a fuel source which could displace other, more costly, fuel sources in the generation of process steam and electricity. Refuse-to-energy plants became a common source of energy.
A common refuse-to-energy facility, generally indicated by reference numeral 2, is shown in FIGS. 1-3. In operation, a crane or a front end loader (not shown) picks up a quantity of refuse from a refuse storage area and deposits it into charging hopper 4. Charging hopper 4 has a large plan area to facilitate this operation and acts as a funnel to feed the refuse to feed chute 6. Refuse travels down feed chute 6 by gravity until it reaches ram table 8 at the bottom of feed chute 6. Ram feeder 10 pushes refuse from ram table 8 horizontally onto reciprocating grates 12 for incineration in combustion zone 14 of furnace 16.
As depicted in FIG. 2, combustion zone 14 or the lower furnace environment is bounded by furnace walls 18 which include a plurality of horizontally spaced boiler tubes 20 and tube joining members or membranes 22 which structurally join adjacent boiler tubes 20 to one another. Boiler tubes 20 carry cooling water to recover the heat given off from the burning of refuse in combustion zone 14.
As previously described, boiler tubes 20 are subject to corrosion due to the corrosive constituents in refuse, which may include sodium, sulfur, potassium, vanadium, chloride, lead and zinc. Further, combustion zone 14 is constantly changing between an oxidating atmosphere (an excess of O.sub.2 beyond that need for combustion) and a reducing atmosphere (a deficiency of O.sub.2 below that needed for combustion) which can rapidly accelerate corrosion. Therefore, some form of corrosion protection is need.
Experience has dictated that boiler tube protection for at least the furnace front wall and the side walls is required up to a height of about thirty feet above the grate surface where there is reasonable assurance that oxidation zones are predominant. In a typical arrangement, the lowest part 21 of the protective wall structure, usually the bottom three feet, is comprised of thick refractory firebricks. This is necessary in this area 21 to protect the boiler tubes from the intense local combustion temperatures and corrosive gases. As previously mentioned, prior art protective wall structures used to protect the boiler tubes in the area 23 above the thick firebrick to a height of about thirty feet have been costly and unreliable and have exhibited a relatively short useful life.
While protective structure for the boiler tubes in area 23 is necessary to prevent costly tube failures, it is equally important that the protective structure have a high thermal conductivity rate. A protective structure with a low thermal conductivity rate reduces the effectiveness of the water-cooled surface it is protecting by preventing the heat given off from the refuse combustion to reach the boiler tubes. Therefore, it would be desirable to have a protective wall structure in area 23 for preventing the boiler tubes which is inexpensive, reliable and which has a high thermal conductivity rate.
A prior art design of a furnace wall protective arrangement 24 is conveniently described with reference to FIG. 4. Furnace wall 26 includes boiler tubes 28 with a large quantity of pin studs 30 attached thereto and membranes 32 which join adjacent boiler tubes 28. A sprayed on or hand troweled castible refractory 34, typically a silicon carbide refractory, is applied to the interior of furnace wall 26 to protect boiler tubes 28. Pin studs 30 increase the heat transfer between furnace interior 36 and enhance the mechanical bond between boiler tubes 28 and castible refractory 34.
One drawback of arrangement 24 is that refractory 34 has proved to be an insulator against heat transfer. Further, refractory 34 has been susceptible to breaking or chipping, a.k.a. spalling, mostly due to the mechanical pressure associated with thermal expansion and contraction. If the refractory spalls during operation, boiler tubes 28 are left unprotected from the gases and the flames in combustion zone 14. Unprotected boiler tubes exposed to combustion zone 14 corrode and leak, frequently leading to the premature shut-down of the entire unit for repair.
Another drawback of arrangement 24 is that refractory 34 must be properly applied and cured during installation to achieve its expected quality and physical characteristics. A lack of quality control during the refractory installation will result in a lower quality protective refractory. A lower quality refractory leads to accelerated spalling and deterioration, and thus also leads to accelerated tube failure. Therefore, to properly apply refractory 34 and achieve its desired characteristics, a high quality control over refractory installation is required. This required quality control increases the installation cost of the protective wall structure.
An additional drawback associated with arrangement 24 is that it requires pin studs 30 to create the required mechanical bond to refractory 34. The use of pin studs 30 on boiler tubes 28 increases installation and product costs over units having boiler tubes without studs.
In another prior art design, not shown, relatively thin silicon carbide tiles are used in lieu of the castible material. The tiles are attached to the boiler tubes by a layer of mortar. Mortar is also used to fill the small gaps between adjacent tiles. However, because of the thermal expansion which occurs when the furnace is brought up on line after being taken down for an outage, adjacent tiles expand into each other, crack, and subsequently fall off the wall. The fallen tiles leave the boiler tubes immediately therebehind exposed to, and unprotected from, the furnace interior environment. As previously mentioned, unprotected boiler tubes exposed to the furnace interior corrode and leak.
Therefore, it would be desirable to have a boiler tube wall protective structure that would serve the dual functions of heat transfer and protection from conditions found in the combustion zone of the furnace. Further, it would be desirable to have a long-lasting boiler tube wall protective structure which prevents unscheduled facility outages due to boiler tube failure and does not require frequent repair.